In recent years, the development of electronic paper has been promoted in companies, universities, etc. Applied fields expected to utilize electronic paper have been proposed, including a variety of fields, such as electronic books, a sub-display of mobile terminal equipment, and a display part of an IC card. One promising method of electronic paper is that which uses a cholesteric liquid crystal. A cholesteric liquid crystal has excellent characteristics, such as the ability to semipermanently hold a display (memory properties), vivid color display, high contrast, and high resolution.
As for the multi-gradation display method by cholesteric liquid crystal, there have been proposed various driving methods. The method of driving a multi-gradation display with a cholesteric liquid crystal is divided into a dynamic driving method and a conventional driving method.
Japanese Laid-open Patent Publication No. 2001-228459 describes a dynamic driving method. However, the dynamic driving method uses complicated drive waveforms, and therefore, requires a complicated control circuit and a driver IC and also requires a transparent electrode of the panel, having low resistance, resulting in a problem that the manufacturing cost is increased. Further, the dynamic driving method has a problem that power consumption is large.
Y.-M. Zhu, D-K. Yang, Cumulative Drive Schemes for Bistable Reflective Cholesteric LCDs, SID 98 DIGEST, p 798-801, 1998 describes a conventional driving method. This Non-patent document describes a method of driving the state gradually from a planar state to a focal conic state, or from the focal conic state to the planar state at a comparatively high semi-moving picture rate by making use of the cumulative time inherent in liquid crystal and adjusting the number of times of application of a short pulse.
However, in the driving method described in this non-patent document, because of such a high semi-moving picture rate, the drive voltage is as high as 50 to 70 V, and this is a factor that increases the cost. Further, the “two phase cumulative drive scheme” described in this non-patent document 1 uses the cumulative times in two directions, i.e., the cumulative time to the planar state and the cumulative time to the focal conic state using the two stages, i.e., the “preparation phase” and the “selection phase”, and therefore, there is a problem of display quality. Further, a fine pulse is applied a number of times, and therefore, the driving method described in this non-patent document has a problem that power consumption is large.
Japanese Laid-open Patent Publication No. 2000-147466 and Japanese Laid-open Patent Publication No. 2000-171837 describe a method of driving a fast-forward mode that applies resetting to the focal conic state. This driving method has an advantage that a comparatively high contrast can be obtained compared to the above-mentioned driving method. However, the writing after resetting requires a high voltage that is difficult to achieve with a general-purpose STN driver, and further, the writing is cumulative toward the planar state, and therefore, the crosstalk to the half-selected or non-selected pixel becomes a problem. In addition, this driving method also has a problem that power consumption is large because a fine pulse is applied a number of times.
When a gradation is set by making use of the cumulative time using the conventional driving method, a method of varying the pulse width has been conceived, in addition to adjusting the number of short pulses as described above. Varying the pulse width is more advantageous than adjusting the number of times short pulses are applied from the standpoint of suppression of power consumption. Hereinafter, the method of setting a gradation by varying the pulse width to change the cumulative time is referred to as a PWM (Pulse Width Modulation) method.
Japanese Laid-open Patent Publication No. 04-62516 describes a configuration in which a positive polarity pulse and a negative polarity pulse having different pulse widths are applied to a liquid crystal display device, although the display device does not use a cholesteric liquid crystal.